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Chicagoland MG Club: Driveline
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  Chicagoland MG Club:Driveline
Just a Simple Wrench

by Ann & Jake Snyder

Not Really Fixed,
But Working

Every once in a while a repair fits someplace between "emergency" and "by the book". There are a lot of reasons for these intermediate repairs, mostly having to do with time, money and the inconvenience involved in making a permanent repair. One example of "not really fixed, but working" came as the result of the ignition light on the '72 GT going on for about two seconds while running at about 40 mph. The light was bright and then went out, and the car kept going as if nothing had happened. It was even hard to believe that the light going on was not imaginary. After all, if the ignition light goes on, the car usually stops. And if it does keep running, and if the alternator bearings have locked up, there is almost immediately a scream from the fan belt as it is burned apart by being rubbed over the frozen alternator pulley. The only other common event is when the light goes on dully at first, then grows to full brilliance in a few miles as more of the rectifier diodes fail. We have never had a regulator that failed, though we have replaced them from time to time when the case seemed too rusty for comfort. Perhaps the on and off act was the regulator. The other of us was driving the car when the ignition light did the same on and off trick, and we decided that the best time to check on the problem was when the GT was kind enough to make it back home by itself. The only clue the car gave was a flickering from the ignition light as the engine stopped turning after it was turned off.

Now, we always have a good spare alternator in the extra battery box that resulted when we changed from twin six volt batteries to a single twelve volt. But did we really want to go ahead and use it? We had a lot of other things to do, and rebuilding an alternator completely did not fit into the schedule. Maybe the problem was just the regulator, and these are held in place with a single screw. So the whole repair could take a little as five minutes to remove the alternator, five minutes to change the regulator, and five minutes to replace the unit and tighten everything properly. And that way, we would still have our good spare. Besides, we do not have a test rig, and anytime we repair an alternator, it must go back on the car to find out if it really works. This sounds complex, but to us it really is preferable to taking the alternator to have tested and/or repaired by someone else. And we knew that we had the common parts to repair most alternator ills. These common parts are the regulator, rectifier, and brushes. The important point about the replacement rectifier is that it must match the jack on the wiring harness. All of ours are big-big-little, but the alternative is little-big-little. If this does not make sense, look at the back of the alternator until you see it. It could be compared to looking at the night sky until the Little Dipper becomes obvious. The big-big-little type was fit to early (69-74) and late (79-80) MGBs.

These parts can be replaced very easily. We removed two screws securing the back cover and immediately noted that there was a fair amount of corrosion on the regulator case and on the aluminum housing, so perhaps we had some bad connections, as well. And as long as the back was open, we decided to check the brushes. At the princely sum of two dollars a set, the possibility that the brushes might be worn down enough to matter was worth checking. The brushes run on the end of a thick disk which has one contact in the middle and the other around the periphery. Both are connected to the rotor winding by thin wires that are channeled through grooves in the shaft under the rear bearing and are soldered to connections on the slip ring. But we did not have time to remove the slip ring - if the repair went that far, the spare alternator would have to be used until we had a little more time. The point to remember is that the brush in the center wears more slowly than the brush that contact the periphery, which means that if the two brushes are much different in length, it is time to replace them.

Before disconnecting the various leads to the brushes, we made a drawing so we could connect the leads to the right places. There are a lot of drawings of where the leads should go in various books, but the insides of alternators vary immensely, with two types of regulators (one has a case ground, the other has a black lead), a suppressor capacitor and a voltage-limiting diode as possible complications. The illustration you find may not be what you actually have. The screws that hold the brush assemblies in place were removed, and a problem became clear - the springs should have been strong enough to make pushing the brushes into the holders a little painful. These springs were very soft.

That led us to check a little farther to see if the slip ring was still okay. We could have removed the brush holder, but our five minutes to repair the alternator were ticking away. Instead, we used a small-bulbed, flexible lamp sometimes referred to as a bore lamp. (Due to its vague resemblance to an instrument used by physicians for internal examinations, however, it is frequently the butt of many jokes.) The tiny bulb revealed a black spot on the peripheral contact, and it was likely this imperfection coupled with the weak brush springs that caused the problem. These defects were also consistent with the flickering from the ignition light when the engine was shut down. It was also clear that the slip ring was deeply grooved a replacement was in order at the earliest convenience.

So we avoided another call for emergency road service and still have both our spare and can look forward to an enjoyable afternoon of properly repairing the alternator. And the not-really-fixed alternator worked again.

©2001 Chicagoland MG Club, All rights reserved.